Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semi-conducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1s−1). In addition, it is important that the semi-conducting material is relatively stable to oxidation, i.e. it has a high ionization potential, as oxidation leads to reduced device performance.
A known compound which has been shown to be an effective p-type semiconductor for OFETs is pentacene [see S. F. Nelson, Y. Y. Lin, D. J. Gundlach and T. N. Jackson, Appl. Phys. Lett., 1998, 72, 1854]. When deposited as a thin film by vacuum deposition, it was shown to have carrier mobilities in excess of 1 cm2 V−1 s−1 with very high current on/off ratios greater than 106. However, vacuum deposition is an expensive processing technique that is unsuitable for the fabrication of large-area films.
Regular poly(3-hexylthiophene) has been reported with charge carrier mobility between 1×10−5 and 4.5×10−2 cm2 V−1 s−1, but with a rather low current on/off ratio between 10 and 103 [see Z. Bao et al., Appl. Phys. Lett. 1997, 78, 2184]. In general, poly(3-alkylthiophenes) show improved solubility and are able to be solution processed to fabricate large area films. However, poly(3-alkylthiophenes) have relatively low ionisation potentials and are susceptible to doping in air [see H. Sirringhaus et al., Adv. Solid State Phys. 1999, 39, 101].